The prior art has recognized two types of adhesive compositions; hot melt-type adhesive compositions and reactive-type adhesive compositions.
The hot melt adhesive compositions provide instant adhesive strength upon cooling after bonding so that the workability of the bonded product is excellent. However, they have disadvantages of low adhesive strength at elevated temperatures because of their rheological characteristics.
In order to obtain high heat adhesive strength at elevated temperatures, it is conventional to use reactive or curable adhesive compositions. However, the initial adhesive strength of such compositions is weak because the time for reactive curing is too short, consequently high instant adhesive strength cannot develop as with hot melt-type adhesives. Accordingly, when the reactive adhesives are used for the preparation of multi-layered sheets such as corrugated boards or plied boards, the handling characteristics, during manufacture, are poor.
The hot melt-type adhesive compositions generally have 100% of solid component. They are applied in a melted condition (in general at temperatures in the range 160.degree. - 200.degree.C.) They instantly bond the two substrates upon cooling at a temperature lower than the melting point of the composition.
The thermophysical characteristics of each component of the composition controls the viscosity of the melted adhesive composition as well as the final physical strength of the adhesive resin and the characteristics of temperature dependency. Accordingly, it has been required to select the adhesive characteristics from the viewpoint of the melting temperature, the final physical adhesive characteristics and the temperature dependency required in each application.
The viscosity of melted adhesive compositions are closely limited by the requirement of coating workability though it depends upon the type of coater that is used. In order to give a suitable coatable viscosity of the melted adhesives, each of the components should have a relatively low melting point and softening point. To meet these requirements, it has been required to increase concentration of additives of low molecular weight material which have high temperature dependency and low adhesive property, such as wax.
These requirements for the conventional hot melt-type adhesive compositions result in inferior physical strength and a temperature dependency characteristic during the application procedures. It has been proposed to decrease the viscosity of the melted adhesive composition by rising the temperature during coating. However, the thermal stability of the components of the melt adhesive compositions is limited, begins to rapidly degrade at about 200.degree.C. It has thus been difficult to modify the melted viscosity, the physical strength or the temperature dependency of such prior art melt adhesives. Moreover, in order to properly heat to such high temperatures at the time of coating, it has been difficult to use chemically-active ingredients as they tend to become thermally unstable, so that only low adhesive strength is possible because of weak adhesives thus available.
Non-solvent type reactive adhesive compositions have high reactivity at elevated temperatures so that such compositions have a pot-life that is too short. For this reason reactive adhesive compositions are coated at about room temperature. For this reason, it has been necessary in order for such compositions to have coatable low viscosity at room temperature, that the substrates be bonded with a monomer or a prepolymer having a low molecular weight. With such monomers or prepolymers the cohesive properties have been too low to impart a suitable instant adhesive force. Solvent-type reactive adhesive compositions have been used wherein a high molecular weight compound having an effective reactivity or a mixture of reactive low molecular weight compounds and another reactive component to form the reactive composition is dissolved in a solvent. The solution is mixed with a hardener solution and this mixture is coated and slightly dried (by hot-air, etc.) and the coated substrates are bonded in a viscous condition. Even here the instant adhesive strength at the time of bonding is too low. Various other types of adhesives such as emulsion-type and solvent-type adhesives, have been tried but suitable adhesive strength can be imparted only after removing water or the solvent. The final adhesive strength here too is relatively low. The characteristics of the prior art conventional adhesive compositions are shown in Table 1.
TABLE 1 ______________________________________ Type of Pot-life Instant Applicable Adhesive for coating adhesive environment strength temperature (room temp. to 60.degree.C.) ______________________________________ Commercially available hot melt-type adhesive Non-solvent-type reactive adhesive Solvent-type reactive adhesive Solvent-type non-reactive adhesive ______________________________________ ( .... good, .... bad)
It can been seen from Table 1 that the prior art has provided no adhesive composition having the three important functions of (a) pot-life for coating, (b) an instant adhesive strength at a room temperature to 60.degree.C. and (c) an applicable environment temperature. It is clear, from the above description of adhesive compositions, that it has been difficult to impart these three important functions.
The conventional adhesive compositions previously available have not been suitable for the preparation of polymeric corrugated boards, film-laminated sheets, plied boards or other multi-layered sheets, which requires during manufacture, a high instant adhesive strength and also high final adhesive strength between the layers of such sheets.